FIG. 1 is a simplified block diagram showing a signalling path of a cellular telephone. An antenna 1 is coupled to a radio frequency (RF) block 2. Antenna 1 both receives and transmits RF signals for accomplishing a telephone conversation. A Baseband Signaling Circuit (BSC) 3 is interposed between the RF block 2 and a controller, typically implemented as a microprocessor based controller (microcontroller) 4. BSC 3 outputs an analog signal (Tx) 2a to the RF block 2, Tx 2a being used to modulate a suitable RF carrier. BSC 3 receives an analog signal (Rx) 2b. Rx 2b represents a demodulated RF signal that is received by antenna 1. Communication between BSC 3 and the microcontroller 4 is by digital data, including an Interrupt signal line 3a, an address bus 4a, and a data bus 4b. In operation, the BSC 3 and the microcontroller 4 implement a selected signalling protocol when transmitting and receiving a telephone communication.
A conventional Advanced Mobile Phone Services (AMPS), and also a Total Access Communication System (TACS), signalling protocol and format is described in Appendix A of the Specification.
As described in Appendix A and FIG. 7, for a Forward Control Channel (FCC or FOCC) and a Reverse Control Channel (RCC or RECC) signalling protocol, the FOCC signalling is a continuous bit stream from a base station (or land station) to a mobile station. Forward and Reverse Voice Channel (FVC and RVC) signalling protocols are employed for voice channels (or speech channels). The FVC and the RVC are both burst type messages; that is, not a continuous bit stream.
Referring to FIG. 8, a Supervisory Audio Tone (SAT) and a Signalling Tone (ST) are used on the voice channel, SAT being a continuous signal from base station to mobile station, with the mobile station transponding the SAT back to the base station. The ST is a burst type signal from the mobile station to the base station.
At present, mobile telephones are not available that are operable on both the AMPS and the TACS systems. Thus, if a mobile telephone is constructed or programmed to be an AMPS telephone, it cannot be user-selected to operate also on TACS. But, the same signaling circuit can be used in both types (AMPS/TACS) telephones, and the selection of the signalling circuit may occur at initialization.
The AMPS/TACS selection of the signaling circuit can be achieved by changing the bit rate. The frame structure (both control and voice channel), and the supervisory signals (SAT and ST), are identical for both systems.
However, narrow band AMPS (NAMPS) and narrow band TACS (NTACS) present clearly different signaling protocols. A subaudible signalling protocol is employed on NAMPS voice channels. NAMPS voice channels are referred to as narrow band voice channels, because the channel spacing is substantially smaller than on an AMPS voice channel (30 Khz .fwdarw.10 Khz). Correspondingly similar differences exist in NTACS as compared to TACS.
The NAMPS/NTACS system is a dual-mode system. That is, mobile telephones must be operable both on AMPS/TACS--specific wide band voice channels, and on NAMPS/NTACS--specific narrow band voice channels (utilizing subaudible signaling).
The NAMPS/NTACS narrow band voice channel signalling protocol (frame) is shown in FIG. 3.
Supervisory signalling on the voice channel is realized by DSAT and DST. On voice channel mobile receivers there is utilized a continuous bit stream, Digital SAT (DSAT) 200 bit/s, NRZ-coded, which is also transponded to the base station. There may be up to seven different DSAT patterns.
Furthermore, ST is a Digital ST (DST), also 200 bits/s, NRZ. The DST signal is generated by inverting the transmitted DSAT.
Voice channel signalling is accomplished by a digital DATA WORD, which is 100 bit/s, Manchester coded. The DATA WORD does not contain repeats, as on AMPS and TACS (control and voice channels), but only one data message. The DATA WORD is preceded by a Word Sync pattern (WSYNC) which is a fixed 30-bit pattern, 200 bit/s, NRZ.
DSAT is a continuous bit stream, which can be replaced by WSYNC and DATA WORD for certain periods of time. Thus, the DATA WORD is not a burst type of transmission having its own frequency, but is instead "embedded" in the DSAT pattern.
Based on the foregoing, required NAMPS signalling functions for a mobile station include the following:
a) detecting received DSAT (NRZ) and transponding DSAT to the base station; PA0 b) detecting WSYNC (NRZ) and DATA WORD, Manchester-decoding DATA WORD, and also BCH-decoding DATA WORD; PA0 c) transmitting DST (NRZ) (invert transmitted DSAT); and PA0 d) transmitting WSYNC (NRZ) and DATA WORD, DATA Manchester and BCH-coded. PA0 a) The bit rate is substantially different, i.e., significantly slower. PA0 b) DSAT, DST and WSYNC are not Manchester-coded, but DATA WORD is, although all of these signals appear in the same continuous bit stream. Thus, DATA WORD must be encoded (transmitter) and decoded (receiver), but DSAT and WSYNC not encoded (transmitter) or decoded (receiver). PA0 c) DATA WORD does not contain repeats. As a result, AMPS 3/5 majority voting is not required. PA0 d) DATA WORD is "embedded" in the DSAT bit stream, and occasionally replaces the DSAT bit stream. PA0 e) WSYNC has a different length (30 bits instead of 11 bits as found in AMPS). PA0 f) Also, there are seven different DSAT patterns, instead of the three SAT patterns.
As a result, several problems must be overcome to realize NAMPS signalling in a mobile station, as compared to an AMPS signalling embodiment. These problems include the following.
One known implementation of NAMPS narrow band voice channel signalling includes a conventional AMPS (TACS) signalling circuit, and also additional circuitry to accomplish NAMPS signalling, to and from the microcontroller 4 or some other controller. This approach also requires a substantial amount of additional software for the microcontroller 4.
By example, this technique can be realized by using a commercial AMPS-signalling circuit, such as the DPROC/UMA 1000, available from Philips/Signetics, for AMPS signalling, and also an additional circuit, by example a NE5234, a switch, and a microcontroller (PCB80C552) to accomplish narrow band NAMPS signaling.
However, this approach requires additional components, which increases at least component cost and also power consumption. Depending on the amount of additional circuitry to implement NAMPS, a substantial amount of additional software may also be required. Also, surface area requirements on printed circuit boards is increased, thus making it more difficult to implement a small, lightweight portable telephone.
It can thus be realized that what is required is a signalling circuit that is both area and cost effective, consumes no additional current, has an efficient interface to microcontroller software, and is operable on both AMPS/TACS and NAMPS/NTACS signalling systems.